The present invention relates to a method for growing a thin film by using a plasma deposition apparatus, and more particularly to a film production method suited to grow a silicon-including film, that is, a film containing silicon as a single constituent element or one of constituent elements, such as a monocrystalline silicon film, polycrystalline silicon film, amorphous silicon film, silicide film including silicon oxide film, silicon nitride film, and silicon carbide film, etc. The term "silicide" will mean binary compounds of silicon and another element.
Vapor phase film production methods of various kinds have been known which include the CVD method (namely, chemical vapor deposition method), sputtering method, plasma CVD method, and others. Specifically, the plasma CVD method has advantages that the operation (or treatment) temperature is low and the film production rate (namely, film deposition rate) is large, and therefore will be positively employed in a process for fabricating a semiconductor device.
In the above fabricating process, the plasma CVD method is applied mainly to the production of a silicon-including film, and is important particularly in the following cases.
(A) A silicon nitride (Si--N) film is produced to be used as a passivation (or protection) film of a semiconductor device.
(B) An amorphous silicon (a--Si) film is produced to form a solar cell.
In the case where a thin film is grown by the plasma CVD method, an apparatus for growing the film is required to have (1) a plasma generation part (including an evacuation system), (2) a gas introduction part (for introducing into the plasma generation part a gas containing silicon as at least one constituent element), and (3) a substrate holding part (namely, a substrate stage).
In conventional plasma CVD apparatuses, D.C. (direct current) glow discharge or RF (radio frequency) discharge having a frequency f which lies in a range from tens of kilohertz to tens of megahertz, is used to produce the plasma.
FIG. 1 is a schematic view showing the structure of a plasma CVD apparatus of D.C. glow discharge type. In FIG. 1, reference numeral 1 designates a vacuum chamber, 2 electrodes, 3 a D.C. power supply, 4 a substrate stage, 5 a substrate, 6 a discharge-gas introducing leak valve, and 7 a plasma.
Referring to FIG. 1, a discharge gas is introduced into the vacuum chamber 1 through the leak valve 6, and then a voltage from the power supply 3 causes discharge between the electrodes 2. Thus, a thin film is grown on a surface of the substrate 5 by active species (namely, active particles) produced in the discharge.
FIG. 2 is a schematic view showing the structure of a plasma CVD apparatus of RF discharge type. In FIG. 2, the same reference numerals as in FIG. 1 designate identical or equivalent parts. Further, reference numeral 8 designates a capacitor, and 9 an RF power supply. The apparatus shown in FIG. 2 is different from that shown in FIG. 1 in that a high-frequency power supply, that is, the RF power supply is used in place of the D.C. power supply.
The growth of a film using the conventional plasma CVD apparatuses can be made only under limited operating conditions, since the apparatuses have the following drawbacks.
(a) A gas pressure, at hwich discharge can be generated, usually lies in a range from 10.sup.-2 to 10.sup.-1 Torr, and it is difficult to deposit a film at pressures less than 10.sup.-2 Torr.
(b) In the apparatus of RF discharge type (shown in FIG. 2), an ion sheath is formed in front of the substrate, and thus the substrate is automatically applied with a self-bias voltage V.sub.sb when viewed from the plasma. As a result, ions incident upon the substrate have a kinetic energy corresponding to the self-bias voltage, that is, an energy of hundreds of electron volt or more. It is difficult to make the incident energy of ions less than the above value. The characteristic of a deposited film is readily affected by the bombardment of ions having such a large kinetic energy, and therefore the conventional apparatus which cannot make small the kinetic energy of incident ions, is disadvantageous.
(c) When a film is grown in each of the conventional apparatuses shown in FIGS. 1 and 2, an electrode material (namely, a metal) is sputtered since it is kept in contact with the plasma, and the material thus sputtered is contained in a deposited film as a contaminating material, which has an adverse effect on optical properties of the deposited film.
Further, there arise problems other than the above drawbacks. For example, in the case where a silicon-including film e.g. a silicide film is grown with the conventional apparatuses, SiH.sub.4 serving as a silicon-supplying gas is used as a discharge gas or part thereof. That is, in the case where an Si--N film is grown, a gas mixture containing SiH.sub.4, N.sub.2 and Ar is usually employed. (Refer to a book entitled "Semiconductor Plasma Process Technology" edited by Takuo Sugano and published by Sangyo Tosho, 1980, pages 238 to 242). Further, in the case where an a--Si film is grown, a gas mixture containing SiH.sub.4 and Ar is usually employed as described in a Japanese article entitled "Amorphous Silicon Solar Cell" by Yoshihiro Hamakawa ("Kotai Butsuri" Vol. 14, No. 10, 1979, pages 641 to 651).
However, when SiH.sub.4 is used as a silicon-supplying gas, hydrogen is contained in the deposited film, and thus there arise the following problems.
(a) In an Si--N film used as a protection film, impurity hydrogen atoms decompose to migrate into a semiconductor element, and thus the characteristic of the element is degraded, as described in an article entitled "Threshold-voltage Instability in MOSFET's Due to Channel Hot-hole Emission" by R.B. Fair et al. (IEEE, ED-28, 1981, pages 83 to 94).
(b) In an a--Si film, hydrogen atoms having saturated dangling bonds may dissociate at elevated temperatures higher than 300.degree. C., and desorb from the film. Thus, the density of localized energy states is increased.
In order to grow a silicide or silicon-including film which does not contain hydrogen, a halogenide silicon gas, for example, SiF.sub.4, SiCl.sub.4, SiFCl.sub.3, SiF.sub.3 Cl, SiBr.sub.4, or the like is used as the silicon-supplying gas, in place of SiH.sub.4. However, when the halogenide silicon gas is used as the silicon-supplying gas in the conventional apparatuses shown in FIGS. 1 and 2, a desired film is not deposited on the surface of the substrate, but a silicon wafer used as the substrate is etched. Two main reasons why the film is not deposited are as follows.
(A) In these apparatuses, an operating gas pressure higher than 10.sup.-2 Torr is used, and therefore the electron temperature of discharge is low, that is, about 4 eV. On the other hand, the bonding energy Q of a halogenide silicon gas (for example, Q.sub.Si--F equal to 115 kcal/mol or Q.sub.Si--Cl equal to 67.8 kcal/mol) is greater than that of SiH.sub.4 (namely, Q.sub.Si--H equal to 53.7 kcal/mol) (see JANAF, "Thermochemical Tables", Dow Chemical Co., Midland; Mich.). Accordingly, it is impossible to fully decompose the halogenide silicon gas by the discharge generated in the conventional apparatuses, and therefore a silicon-including film is not grown.
(B) Further, in the conventional apparatus shown in FIG. 2, the energy of ions incident upon the surface of the substrate is large, and therefore a film deposited on the substrate surface is sputtered or decomposed by the incident ions. Thus, the film is prevented from growing.
As can be seen from the above explanation, in order to grow a silicon-including film which is free from hydrogen, a plasma CVD apparatus is required which generates discharge having a high electron temperature and can decrease the energy of ions incident upon the substrate surface. Further, a plasma CVD apparatus in which each of the operating gas pressure, the electron temperature of discharge and the energy of ions incident upon the substrate surface can be varied in a wide range, is very useful for the production of the above film.